Abstract: Biological processes ranging from embryonic development to carcinogenesis rely on the regulation or dysregulation of cell growth and size; despite this central biomedical importance, the mechanisms by which cells know to grow to a certain size remain poorly understood. With few exceptions, virtually all current thinking about cell size control is based on traditional paradigms that focus on signaling networks such as the TOR metabolic pathway, or the cell cycle pathway. However, we lack even a basic mechanistic understanding of the way in which cellular variables such as genomic content (ploidy) and ribosome biosynthetic activity exert their well-known effects on cell size, particularly in the context of cell size homeostasis i multicellular organisms. The fundamental gap in our understanding of this critical biological process underscores the urgent need for new ways of quantitatively looking at cell size control. We recently introduced the novel concept that non-membrane bound ribonucleoprotein (RNP) bodies, including the ribosome-producing nucleolus, are dynamic liquid phase micro-reactors for rapid RNA processing. Our previous studies with other disordered liquid-like bodies suggest that the size of nucleoli should scale with the size of cells. However, basic biophysical considerations suggest that nucleoli of different size will produce ribosomal subunits at different rates; large nucleoli in large cells may not be able to keep up with the ribosome requirements for steady cell growth, providing a biophysical mechanism for cell size feedback and growth control. To test this hypothesis, we will use novel imaging-based assays to measure the function of individual nucleoli of different size, utilizing a biochemically accessible Xenopus oocyte system. We will then examine the consequences of this size- dependent activity, dissecting the relationship between nucleolus size and activity, and cell size, within the multicellular worm C.elegans. Using 3D confocal imaging and custom image analysis algorithms, we will measure the effects of altered nucleolus size and biosynthetic activity on cell size, in both diploid and tetraploid worms. Finally, we will develop theoretical biophysical models for understanding these data based on the size dependence of the rate of ribosomal RNA processing within the dynamic liquid- like nucleolar material. Public Health Relevance: Biological processes ranging from embryonic development to cancer all rely on the regulation or dysregulation of cell growth and size. The nucleolus is a nuclear organelle that plays a well known role in cell growth control. This proposal seeks to identify the biophysical mechanism by which the nucleolus may provide cell size feedback for regulating cell growth, which will have important implications for understanding and controlling physiological and disease driven cell growth processes.